Integrated circuits and other electronically active devices are constructed on wafers, usually crystalline semiconductor wafers, using photolithographic processes used in the microelectronics industry. Each wafer usually includes multiple, identical devices that are separated from the wafer after construction, for example, by sawing. The devices can then be packaged, tested for functionality, and sold.
To avoid expenses associated with sawing and packaging defective devices, devices can be tested while they are still a part of the original wafer in which the devices were constructed. A variety of methods can be used to accomplish this. For example, pads accessible to test probes can be provided on each device, electrical signals can be applied to the pads, and the device can thereby be tested. The device response can then be compared to a device that is known to function properly, and a functionality determination can be made.
Sometimes circuitry can be included on the wafer substrate that is not included in the packaged devices. The circuitry can provide access from an external tester and can also aid in applying test signals to the test devices and in collecting and interpreting the response of devices. Such wafer-scale testing is described, for example, in U.S. Pat. Nos. 4,956,602 and 5,440,241. A method for testing integrated circuits that employs serial shift registers that apply signals to various portions of a circuit and collect output is described in U.S. Pat. No. 4,293,919. The serial shift register can shift test data into the device as well as shift performance data out of the device. An external test device can load the test data into the shift register and analyze the performance data that is shifted out.
One technique used to create relatively small active components, such as integrated circuits, in a larger substrate or wafer is described in “AMOLED Displays using Transfer-Printed Integrated Circuits” published in the Proceedings of the 2009 Society for Information Display International Symposium Jun. 2-5, 2009, in San Antonio Tex., US, vol. 40, Book 2, ISSN 0009-0966×, paper 63.2 p. 947. As described in this paper, small integrated circuits are formed in a wafer and released from the wafer by etching around and beneath the circuits to form separated active components. Because the active components are so small, it may be difficult or impossible to include the relatively large pads for external test probes in the active components. Furthermore, because the circuits are separated from the substrate, it may be difficult to connect additional test circuitry to the active components, and the active components themselves can be too small to incorporate such test circuitry. Moreover, the unpackaged active components can be transferred to a destination substrate by a stamp that transfers many devices at a time, making it difficult to test the devices until they are a part of the destination substrate, at which point replacing a defective active component can be difficult.